The present invention relates to an apparatus and a method for computing an air-fuel ratio control signal based on an output signal from an oxygen sensor that detects oxygen concentration in exhaust gas, for example, in an engine for a vehicle.
Heretofore, there has been known an air-fuel ratio control apparatus provided with an oxygen sensor from which output signal is changed in response to oxygen concentration in exhaust gas, for computing an air-fuel ratio control signal based on the oxygen sensor.
In an air-fuel ratio control apparatus disclosed in Japanese Unexamined Patent Publication No. 7-127505, an output signal from the oxygen sensor is converted into data of air-fuel ratio to obtain an actual air-fuel ratio, and an air-fuel ratio control signal is feedback controlled based on a deviation (error amount) between the actual air-fuel ratio and a stoichiometric air-fuel ratio being a target air-fuel ratio.
However, in such an oxygen sensor, while an output is abruptly changed in the vicinity of the stoichiometric air-fuel ratio, in a region apart from the stoichiometric air-fuel ratio, a change in sensor output relative to a change in air-fuel ratio becomes less since a change in oxygen concentration is small. Therefore, conversion accuracy into air-fuel ratio data is largely degraded even in a small variation of sensor output.
Consequently, in a rich or lean region where the air-fuel ratio is largely deviated from the stoichiometric air-fuel ratio, the error amount of air-fuel ratio is misjudged, resulting in a possibility that air-fuel ratio control performance shall be largely degraded.
The present invention has been achieved in view of the above problems, and has an object to provide an apparatus and a method for controlling an air-fuel ratio of engine that can stably converge the air-fuel ratio to a stoichiometric air-fuel ratio by an air-fuel ratio feedback control using an oxygen sensor and also can ensure stability of air-fuel ratio control even if the air-fuel ratio is largely deviated from the stoichiometric air-fuel ratio.
To achieve the above object, the present invention is constructed such that, when an output signal from an oxygen sensor is within a predetermined range including a value equivalent to a stoichiometric air-fuel ratio, the output signal is converted into air-fuel ratio data, to compute an air-fuel ratio control signal based on a deviation between the air-fuel ratio data and a target air-fuel ratio, while when the output signal from the oxygen sensor is outside the predetermined range, it is judged whether an actual air-fuel ratio is richer or leaner than the target air-fuel ratio based on the output signal, to compute the air-fuel ratio control signal based on the judgment result.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.